Email as a protocol is designed such that emails do arrive, not that they necessarily arrive in a timely fashion. Normally you don't have to think this about this because emails are an asynchronous communication medium. It's usually ok if they arrive a a minute or two later than you expect.

But people who think they are really smart have been implementing login via emailed links, turning something that assumes async into synchronous process. I can't log in until this stupid email arrives. This may take several minutes, or one of the billion things that can go wrong with email has, and I'm just stuck.

I'm trying to buy something, and I can't, and I'm basically going to wander away at this point. This shit costs you money.